Conventionally, a typical positive displacement flow meter has a pair of rotors, there are known a non-circle tooth-type flow meter, a Roots-type flow meter, a birotary-type flow meter and the like. The non-circle tooth-type flow meter (which is called a flow meter having the trademark "OVAL") has a relatively simple structure and a high performance as to output the disadvantage thereof is that the rotors are forced to be rotated with an unequal speed rotation so that it is unavoidable to prevent vibrating noises due to pulsation. Further, the disadvantage of the Roots-type flow meter is that because the rotor itself does not rotate uniformly, it is indispensable to employ a pilot gear, and subsequently a phase adjustment between a pilot gear and a rotor and the assembling work become more complicated. Consequently, the whole structure thereof also becomes complicated. Still further, the disadvantage of the birotary-type flow meter is that it is possible to prevent pulsation by selecting an appropriate twist angle (which is normally provided with a twist angle of 1.5 pitch at the tooth width), but it is unavoidable to employ a pilot gear just like in case of the above Roots-type flow meter. From the theoretical point of view, the birotary-type flow meter can transmit rotation without requiring a pilot gear, but the tooth profile is provided with the so-called second tooth profile so that its slippage is very big, and respective teeth are worn considerably by their mutual engagement. Accordingly, it is substantially indispensable to employ a pilot gear. Further, there exists the defect that the casing diameter is inconsistent occasionally, and the cutting work is troublesome and inefficient due to a different tooth profile curve of a pair of rotors.
The following references corresponding to the foregoing known art can be cited: U.S. Pat. Nos. 2,410,172, 2,243,874, 2,287,716, 1,821,523, 1,965,557, etc. Each cited reference has a pair of rotors to be engaged with each other, wherein the tooth profile of a first rotor is quite different from that of a second rotor. Consequently, transmission of energy between the two rotors is not uniform, and a stronger force of one rotor is applied to the other rotor. In view thereof, the vibration due to mutual engagement of the two rotors is large and it is impossible to avoid wear of teeth of the two rotors.
In order to remove the aforegoing defects of the conventional art, the present invention has been made.